Publications

The repayment hypothesis posits that primary sex ratios in cooperative species should be biased towards the helping sex because these offspring “repay” a portion of their cost through helping behavior and therefore are less expensive to produce. However, many cooperatively breeding birds and mammals do not show the predicted bias in the primary sex ratio. Recent theoretical work has suggested that the repayment hypothesis should only hold when females gain a large fitness advantage from the presence of auxiliary adults in the group. When auxiliaries provide little or no fitness advantage, competition between relatives should lead to sex ratios biased towards the dispersing (non-helping) sex. We examined the benefits auxiliaries provide to females and corresponding offspring sex ratios in the red-backed fairy-wren (Malurus melanocephalus), a cooperatively breeding Australian bird with male auxiliary helpers. We found that auxiliaries provide little or no benefit to female reproductive success or survival. As predicted, the population primary sex ratio was biased towards daughters, the dispersing sex, and females with auxiliaries produced female-biased broods whereas females without auxiliaries produced unbiased broods. Moreover, offspring sex ratios were more strongly biased toward females in years when auxiliaries were more common in the population. These results suggest that offspring sex ratios are associated with competition among the non-dispersing sex in this species, and also that females may use cues to assess local breeding opportunities for their offspring. Keywords Cooperative breeding . Malurus melanocephalus. Local competition . Offspring sex ratios. Red-backed fairy-wren . Repayment hypothesis

Understanding why males of many species exhibit two or more sexual ornaments depends upon identifying both the information conveyed and the intended receiver(s) for each signal. Here we focus on identifying the intended receivers for two sexual signals exhibited by male red-backed fairy-wrens Malurus melanocephalus, extent of nuptial plumage and tail length. In doing so we test the multiple receiver hypothesis, which predicts that each trait is directed toward a different type of receiver (e.g., males vs females). Male red-backed fairy-wrens in nuptial plumage exhibit reversed sexual dimorphism for tail length in the breeding season, when their tails are significantly shorter than those of females or males in eclipse plumage. Using both aviary-based experiments and indices of mate choice and social dominance from a natural population, we found that extent of nuptial plumage and age primarily affected female mate choice and that shorter tails were primarily associated with male:male dominance signaling. The field and aviary studies combined are consistent with the multiple receiver hypothesis, in that each trait appears to be directed primarily to a different set of receivers (plumage for females and tail length for males), though each trait may also signal information to the other set of receivers as well. We propose that sexual selection may favor shorter tail lengths in male red-backed fairy-wrens through social competition mechanisms.

Seed dispersal is critical to the maintenance of healthy terrestrial habitats and to the regeneration of degraded habitats. As anthropogenic deforestation, fragmentation, and defaunation continue, animal-mediated seed dispersal mutualisms are likely to be disrupted, with potentially serious consequences for forest dynamics. In this review, we examine the degree to which disperser sociality may influence seed dispersal outcomes. Available data are sparse and suggest that the relationship is complex, but some basic trends do emerge from the work to date. Degree of sociality, or group size, may increase the distance seeds are dispersed but also can lead to increased clumped distributions. Territoriality and the resulting defense of resources are likely to reduce the scale of dispersal and lead to clumped seed distributions. Nesting and mating behaviors such as male display are also likely to lead to more clumped distributions. Clumped deposition can have either positive or negative impacts on seed fate, depending on microsite quality, which can vary greatly. In all cases, however, there are exceptions and caveats and the one clear finding from this review is that more work is needed on this subject. We suggest that comparative studies which assess seed dispersal services offered by closely related, syntopic species that vary in key behavioral parameters will be most enlightening. We also highlight the recent use of molecular markers as a particularly effective tool to infer the dispersal services of given species, and to assess the consequences for genetic structure of the plants they disperse. Keywords: Seed dispersal, frugivorous birds, social behavior, mating system, territoriality, Janzen-Connell effects

The occurrence of multiple phenotypes within a sex of a single species has long puzzled behavioral ecologists. Male red-backed fairy-wrens Malurus melanocephalus exhibit 3 behaviorally distinct types in their first breeding season: breed in bright nuptial plumage, breed in dull plumage, or remain as an unpaired auxiliary (helper) with dull plumage. The retention of dull plumage by auxiliaries and dull breeders is an example of delayed plumage maturation (DPM), a widespread phenomenon in birds whose costs and benefits are not well understood. At a mechanistic level, DPM might allow dull males either to deceptively mimic females (female mimicry hypothesis) or to honestly signal their subordinate status (status-signaling hypothesis). DPM might function via either mechanism to provide ultimate benefits relative to developing nuptial plumage by increasing reproductive success, survival, or both. In this study, we tested the hypothesis that DPM is related to increased male survival in the red-backed fairy-wren via either female mimicry or status signaling. Aviary-based experiments revealed that dull males were perceived as male, which is consistent with the status-signaling hypothesis but contradicts the female mimicry hypothesis. Further aviary and field-based experiments also revealed that dull males were socially subordinate to bright males and received less aggression than bright males, further evidence for status signaling. However, male survival was not related to plumage coloration or breeding status. These findings indicate that male plumage coloration signals social status but that dull plumage does not afford a net survival advantage, perhaps because plumage color is a conditional strategy. Key words: alternative breeding strategies, delayed plumage maturation, fairy-wren, Malurus melanocephalus, plumage coloration, survival.

In many animals, an individual’s social environment and status can change quickly and unpredictably, suggesting an advantage for signals capable of responding to these changes. However, many signaling traits are fixed for relatively long periods. In birds, for example, most plumage-based signals are constrained by molt and unable to respond to sudden changes in social environment. Soft parts, conversely, are vascularized and may provide an alternative signaling system that can be updated relatively quickly. In this study, the effects of changes in breeding status are documented for a putative plumage-based signal, plumage brightness, and a putative soft part signal, bill darkness, in the red-backed fairywren Malurus melanocephalus. Males that switched from unpaired helper to paired breeder within a breeding season developed significantly darker bills within three weeks of the switch relative to males that did not change status. There was no clear corresponding change in plumage brightness, though the data suggests that changes in plumage brightness are at least possible. These results suggest that bill color is a socially mediated trait capable of responding rapidly to changes in breeding status, and it is proposed that the importance of soft parts as labile signals in birds may be currently under-appreciated

Males of many species can breed in distinct alternative phenotypes; for example, in many birds some males breed in dull plumage while others breed in bright plumage. Because females often appear to prefer brighter males, it is unclear why some males breed in dull plumage. Males in dull plumage might enjoy enhanced within-pair reproductive success if they can gain access to better breeding territories, or they might have relatively high extrapair reproductive success if they are better able to intrude on the territories of other males. To test these possibilities, we examined the reproductive consequences of plumage color in the redbacked fairy-wren (Malurus melanocephalus), a species in which males can breed in either bright plumage or dull plumage or serve as nonbreeding auxiliaries. Male plumage color was distributed bimodally and was loosely associated with age, such that some males molted into bright plumage a year or more earlier than others. Both male phenotypes were cuckolded at similar rates, but bright males sired significantly more extrapair young than did dull males, and this effect was independent of age. Thus, 1-year-old males who bred in dull plumage had low seasonal reproductive success compared with same-aged males who bred in bright plumage. These results suggest that males may not reap any fitness benefits by breeding in dull coloration, compared with breeding in bright plumage, but rather may be constrained to breed in suboptimal plumage by the timing of plumage acquisition. Key words: alternative breeding strategies, delayed plumage maturation, extrapair paternity, fairy-wren, Malurus melanocephalus, plumage coloration.